<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>【案例】改变盒子大小</title>

    <style type="text/css">
        .box {
            height: 20px;
            width: 20px;
            background-color: red;
            position: absolute;
            top: 50%;
            left: 50%;
            margin-left: -100px;
            margin-top: -100px;
        }
    </style>
</head>
<body>
<div class="box">
    小
</div>
</body>
<script type="text/javascript" src="../javascript/case7.2.3.js">

</script>
</html>